The growing scarcity of conventional bauxites suitable for Bayer process production of alumina has generated a search for other sources of aluminum values and other methods of processing the variety of aluminum value sources that exist including low grade bauxites, kaolins, fly ash and other aluminum value containing ores and materials. One such approach involves the chlorination of such aluminum value containing materials followed by direct hydrolysis of the aluminum chloride thus formed, or alternatively, decomposition of the AlCl.sub.3 into alumina which can be used in conventional electrolytic cells.
Energy considerations appear to favor high temperature chlorination as a route to aluminum chloride which can be oxidized to obtain alumina and recover the chlorine values for recycle. The commercially successful titanium chloride pigment process has demonstrated the effectiveness of a recycle process using high temperature chlorination with a solid reducing agent followed by separation of the titanium tetrachloride from the accompanying noncondensible chlorinator gases and purification before oxidation.
Researchers seeking to save energy in the electrolytic reduction step of aluminum production by using aluminum chloride instead of alumina in the cell reduction have found that AlCl.sub.3 purification difficulties have to date dictated that alumina be the starting material for the production of the chloride instead of natural bauxite ores or other aluminum value containing materials which also incorporate substantial quantities of various impurities. Hence there has been a twofold need for a process to produce alumina, both as a material to be chlorinated via a chloride reduction route to aluminum metal and as the starting material for direct reduction in conventional cells. When one considers the huge investment in plants now using alumina as the source of aluminum metal, it is obvious that steps must be taken to provide a continuing source of alumina even as the grade and type of aluminum value sources changes. A process which could provide such a source of alumina while at the same time possessing the capability of providing AlCl.sub.3 for chloride reduction cells would seem to provide an optimum solution to the problem.
Studies have been made and process steps proposed to prechlorinate bauxites or kaolins to remove one or more impurities, usually iron oxides and sometimes titania; frequently using HCl and sometimes a limited amount of chlorine with a reducing agent, frequently carbon monoxide. The attempts to remove iron fall far short of reaching the alumina purity which is necessary for producing "cell grade" material.
Proposals have been made to selectively condense the chlorides from a chlorinator exit gas stream, but it is now well established that the formation of iron-aluminum chloride complexes will invariably defeat attempts at simple selective condensation. Not only has the iron impurity level remained excessive in such attempts, but it is generally found that the concentration of titanium and silicon chlorides in the condensed aluminum chloride is much higher than would be expected from the wide temperature differences in their boiling points and certainly higher than can be tolerated in the electrolytic cells.
Proposals have been made to rectify a molten mixture of chlorides, but the corrosive nature of aluminum chloride makes heat transfer across metal surfaces difficult and the propensity of ferric and aluminum chloride to attack carbon by forming intercalation compounds makes the use of generally more corrosion resistant carbon-containing materials virtually impossible. The low price of aluminas, furthermore, inveighs against the energy expenditure for rectification.
In processes analogous to the chloride titanium pigment process which are proposed to produce pigment-sized aluminas, the jet burners needed are not only complex and expensive but produce a very fine alumina which is difficult to collect and handle. The surface areas are below the high surface areas made from aluminum chloride or hydroxide dehydration as in the Bayer process, but the particle size is generally too small for easy handling and cell introduction.
Finally, efforts to recycle chloride values from the impurities are faced with expensive separate oxidation equipment and a silica particle size problem even worse than that of the alumina oxidation reactors.
It is clear from the foregoing that a simple alumina producing analogue of the chloride pigment/chlorine recycle process is neither desirable nor commercially realistic. Aluminum chloride is too near ferric chloride in physical properties, too difficult to separate from ferric chloride, too different in its oxidation properties while the alumina to be produced is too inexpensive a commodity to make a direct analog of the titania process economically viable.